In the manufacture of carbon black, natural gas is combusted with a stoichiometric excess of oxidant, typically air, to provide a hot combustion gas. Oil feedstock is sprayed into the hot gas causing it to pyrolyze into elemental carbon product. This product is carried in the reactor tail gas through various quench steps and heat recovery before it is separated in a filter device. Residual, low pressure tail gas from the filters contains large quantities of nitrogen, water, carbon dioxide, hydrogen and carbon monoxide yielding a low BTU(50-150 British Thermal Unit/standard cubic foot BTU/SCF) fuel. Some tail gas is used to fuel carbon dryer furnaces and boilers, but excess gas is incinerated.
Effective utilization of the low BTU excess tail gas can result in an overall energy efficiency improvement for the carbon black process. Efficiency improvements can also result from effectively integrating waste heat recovery into the carbon black process and limiting quench requirements. Increased efficiency can translate into reduced natural gas requirements or increased carbon black production.
Better utilization of carbon black reactor tail gas has been proposed by several patents in the prior art.
In U.S. Pat. No. 4,261,964, Scott, IV et al. proposed extracting the combustible components (hydrogen and/or carbon monoxide) from the tail gas and replacing 33% to 100% of the natural gas fuel with these components. The method for CO extraction described was a liquid adsorbent process known as COSORB. Hydrogen was recovered from the resultant CO--free tail gas with a cryogenic process.
There are several disadvantages to this process. The solvent CO absorption process (COSORB) is very sensitive to oxygen and water content in the tail gas. Water must be removed from the tail gas with dryers; failure to remove water results in severe corrosion problems in the COSORB equipment due to HCl formation. Oxygen will result in solid precipitates that foul and plug equipment.
The low pressure tail gas (near atmospheric) will require a large absorber column and high solvent circulation rates due to low separation driving force for CO removal. The COSORB solvent requires about 100,000-150,000 BTU/lb mole CO for regeneration. The combination of high energy requirements and high capital makes this recovery method uneconomical.
U.S. Pat. Nos. 4,460,558 and 4,393,034 utilize oxygen enriched air for the carbon black reactor oxidant gas. This minimizes the nitrogen content of the tail gas, upgrading its heating value, and making it suitable for reactor fuel.
Oxygen enrichment also has several disadvantages especially when retrofitting an existing carbon black process. Oxygen enrichment increases combustion chamber temperatures and requires a refractory changeout or the advantages of oxygen enrichment are limited. Oxygen is costly--its cost is about equivalent to the natural gas fuel cost that is replaced. Equipment must be installed to cool and condense water from the tail gas to improve its heating value. The integrated SMR process can retrofit existing carbon black processes without requiring equipment changeout in the carbon black process.
Chen cites in U.S. Pat. No. 4,490,346 a method for using the low BTU content tail gas by combusting with with near stoichiometric amounts of air and then tempering the combustion mixture with diluent tail gas or air before oil injection. A special compact reactor configuration with means for diluent introduction is needed to carry out this process.
The disadvantages of the prior art set forth above have been overcome by the present invention which will be described in greater detail below.